Field of the Invention
The present invention relates to shredder machines and similar crushing, pulverizing and grinding machines, and more particularly to an improved screen used for them.
Related Art
Shredder machines which use screens in close proximity to rotors have been used to break down post-consumer materials and other materials, particularly including plastic materials, such as polyethylene (PE) and polypropylene (PP). The screens are generally curved to maintain a close proximity to the cutters or blades of the spinning rotors and are generally smooth or flat on the side facing the rotor/cutter assembly. It is a known problem for the post-consumer materials to get caught in the screen and to clog the screen. This clogged screen can result in increased friction which generates additional heat and causes additional wear on the cutters and typically requires the shredder machine to be shut down so that the clogged materials can be cleared from the screen. Additionally, clogged screens can reduce the throughput of the machine. Previous screen designs have sought to solve the problem of clogged materials by using grooves on the inner side of the screen facing the rotor assembly that are aligned with the cutters on the rotor such as described in U.S. Pat. No. 6,305,623. When the rotor's cutters pass through the grooves, they further cut and force out any material that is caught up in the screen.
Although this previous screen design allows the rotor to clean the screen as the cutters pass through the grooves, the screen is not designed to help prevent the material from getting caught by the screen in the first instance. To use the cutters for cleaning out the materials that get caught in the screen, additional force and corresponding power is required for the rotor assembly and causes reduced throughput. This previous design also results in high stress zones at the grooves which prematurely wear down the cutters and the screen which then require servicing. The screens for most shredder machines are fabricated from metal plates in which the holes are cut or punched through the plates, and the screen is supported by a series of ribs. When material is caught in the screens, the screens can deflect and cause premature dulling of the cutters and wear on the screen, sometimes leading to the failure of the screen. It would be better to create screen designs that avoid materials becoming caught in the first instance so that additional power is not necessary and premature dulling of the cutters is prevented and the wear of screen is reduced.
Another problem with previous designs which permit materials to clog screens is the possibility of overheated gearboxes along with the additional power that is used to force the rotor to continue grinding the material. To avoid the overheating of the gearboxes, the throughput of shredders is limited because of the current screen designs which permit materials to get caught. Even without overheating the gearboxes, the increased heat of these prior art systems increases the frequency in which the gearbox oil must be changed as compared with a screen that is much less prone to getting clogged.